The prosecutors submitted a search warrant to raid the Seoul office of the Unified Progressive Party over alleged fraud in the party primary, but party officials, including interim leader Kang Ki-kab, refused to let them in.

"The party is taking steps to reform itself," said Lee Jeong- mi, a spokeswoman for the party's emergency committee. "The raid violates the party's basic rights to engage in political activities."

The standoff coincides with a growing fracas over vote-rigging attempts before the April parliamentary elections, where the fledgling left-ring party won 13 seats in the 300-member parliament.

The fraud came to light after an in-house investigation, which forced the four core leaders of the party to resign and sparked infighting among rival factions.

Last week, the emergency committee issued an ultimatum to two of its lawmakers-elect to give up their seats by Monday, signaling it could force them out if they do not comply with the decision.

Lee Seok-gi and Kim Jae-yeon, who ran as proportional representatives in the rigged primary, have refused to step down.

They are known for pro-Pyongyang views allegedly shared by a major faction within the party, which refuses to shoulder responsibility for claimed irregularities.

The leftist party was founded less than a year ago through a merger among three progressive parties. Its new-found status as the third largest party in the parliament was considered a major success.

With even the party's traditional supporters voicing distrust of the party's handling of the controversy, observers say the beleaguered party might eventually split into splinter groups.